Where does Info in return value {ok, Child, Info} for supervisor:start_child come from?

Question

I see from the Erlang docs that supervisor:start_child can return two different non-error results: {ok, Child} and {ok, Child, Info}. Where does this Info get set? Is it different for simple_one_for_one sups as opposed to others? I haven't been able to find an example / documentation on this...

Answer 1

Info comes from the function that starts the child process (as given in the child specification in the supervisor). Most of the time, that function will eventually call gen_server:start_link/4 which only returns {ok, Pid} and never {ok, Pid, Info}, so that case only applies when you have a custom function for spawning the process, probably using the functions in the proc_lib module.

Processes that don't use a pre-defined behaviour are called "special processes" and are described in the OTP Design Principles User's Guide.